Local Bandgap Measurement of Core/Shell Ge/GeSn Nanowires Employing Electron Energy Loss Spectroscopy in Scanning Transmission Electron Microscopy

Group IV Ge_1-xSn_x alloyed semiconductors have attracted a lot of attention because of their numerous applications in the field of monolithically integrated optoelectronic and photonic devices compatible for use on the Si platforms. Such applications include free-space data communications, night-vision sensors, biosensors, and materials for thermal imaging, which dominate in the short-wave infrared (SWIR: 1.5-3 um) and mid-wave infrared (MWIR: 3-8 um) and long-wave infrared (LWIR: 8-14 um) wavelength ranges [1], [2].<br/><br/>Core/shell Ge/GeSn structured nanowires (NWs) exhibit high tunability of electronic properties by varying two parameters, Sn content and lattice strain. Furthermore, these free-standing core/shell Ge/GeSn NWs grown on Si wafers tend to accommodate more elastic strain which is relaxed at the sidewall facets of the wires. In this research, a GeSn shell has been grown around the colloidal-nucleated Ge core using the chemical vapour deposition (CVD) technique. The variation in Sn content incorporated in GeSn alloy strongly affects the electronic bandgap energy and optical transitions, such as the plasmonic oscillations and interband transitions. By increasing the Sn content in the Ge_1-xSn_x shell, the bandgap is expected to experience a transition from indirect to direct bandgap, progressively narrowing down. For high enough Sn concentration, the electronic structure would eventually exhibit an effective negative bandgap, essentially an indirect semimetal behaviour [3].<br/><br/>Here we investigate the behaviour of electronic band structure and bandgap energy demonstrating the potential of high-resolution electron energy loss spectroscopy (EELS) in a monochromated scanning transmission electron microscope (STEM). Spatial locality is essential in the bandgap measurement of these NWs as EELS can provide a wealth of information on various features in the low-energy range, showing the effect of localized defect states that may be present in the bandgap energy onset. Other transitions, such as collective plasmon oscillations and single-electron transitions that depend upon the position of electronic levels within the band structure, are observed and will be discussed. [4]<br/><br/>References:<br/>[1] Moutanabbir, O., Assali, S., Gong, X., O'Reilly, E., Broderick, C. A., Marzban, B., ... & Nam, D. (2021). Monolithic infrared silicon photonics: the rise of (Si) GeSn semiconductors. <i>Applied Physics Letters</i>, <i>118</i>(11), 110502<br/>[2] Luo, L., Assali, S., Atalla, M. R., Koelling, S., Attiaoui, A., Daligou, G., ... & Moutanabbir, O. (2022). Extended-SWIR Photodetection in All-Group IV Core/Shell Nanowires. <i>ACS Photonics</i>, <i>9</i>(3), 914-921.<br/>[3] Assali, S., et al. "Growth and optical properties of direct band gap Ge/Ge0. 87Sn0. 13 core/shell nanowire arrays." Nano letters 17.3 (2017): 1538-1544.<br/>[4] Acknowledgment: This work was carried out at the Canadian Centre for Electron Microscopy, a national facility supported by the Canada Foundation for Innovation under the MSI program, McMaster University, Defence Canada (Innovation for Defence Excellence and Security, IDEaS), and NSERC.